The present invention relates to metal-graphite brushes which are used in electrical motors for automobiles, etc, and in particular, Pb-less metal-graphite brush.
Metal-graphite brushes have been used as brushes for low-voltage operation, such as brushes for electrical motors in automobiles. They are produced by mixing graphite and a metal powder such as copper powder, molding and sintering the mixture. As operated at low voltages, their resistivities are lowered by adding a low resistance metal powder. A metal sulfide solid lubricant, such as molybdenum disulfide or tungsten disulfide, and Pb are added to metal-graphite brushes in many cases. For example, in brushes for heavy load such as brushes for starting motor, Pb and a metal sulfide solid lubricant are added in most of the cases.
In recent years, Pb has been attracting greater attention as one of materials damaging to the environment, and there is a growing demand for Pb-less brushes. Of course, brushes containing no lead have been available up to the present and they have been used in some motors other than starting motors. Even some brushes for starting motors can be used by simply eliminating Pb from them, provided that they are used under normal service environments. To improve the lubricating properties without Pb, Japanese Patent Opening Hei 5-226048 (U.S. Pat. No. 5,270,504) proposes that a metal having a melting point lower than that of copper is mixed in such a way that copper and the metal do not form an alloy. The present inventors, however, found that in metal-graphite brushes wherein a metal sulfide solid lubricant is added to copper and graphite, the elimination of Pb results in an increase in the lead connection resistance under high temperature or high humidity.
The initial object of the present invention is to control the increase in the lead connection resistance of a Pb-less metal-graphite brush even under high humidity.
A secondary object of the present invention is to control, in addition to the increase in the lead connection resistance, the increase in the resistivity of the brush body under high humidity.
Another secondary object of the present invention is to control the increase in the lead connection resistance by means of a small amount of indium.
In the present invention, a metal-graphite brush comprising a copper-graphite brush body to which a metal sulfide solid lubricant is added and a lead embedded in the copper-graphite brush body is characterized in that indium is at least added to an interface between said brush body and the lead.
Preferably, indium is added in a concentration of 0.4-8 wt % substantially in all over said brush body.
Preferably, indium is added in a neighborhood of the lead in the brush body and that no indium is added in a neighborhood of a portion of the brush body with which a commutator of a rotational electric armature is to be in contact.
Preferably, an indium source is provided at least at a portion of the lead embedded in said brush body so as to supply indium to the interface between the brush body and the lead.
Preferably, the metal sulfide solid lubricant is at least a member of a group comprising molybdenum disulfide and tungsten disulfide and a concentration of the metal sulfide solid lubricant is from 1 to 5 wt %.
Preferably, the lead is a non-electroplated copper lead.
According to the experiments by the present inventors, the increase in the lead connection resistance under high humidity is attributed to the influences of the metal sulfide solid lubricant. When the metal sulfide solid lubricant was not added, the lead connection resistance did not increase substantially even under high humidity. This is related to the presence or absence of Pb. When Pb was added, the lead connection resistance hardly increased. In Pb-less brushes, in correspondence with the increase in the lead connection resistance, the copper powder and the embedded lead in the brush body showed a greater tendency to be oxidized under high humidity.
The metal sulfide solid lubricant such as molybdenum disulfide or tungsten disulfide is added by the designer of the brush, but the metal sulfide solid lubricant is indispensable to brushes so as to have a long service life. Without metal sulfide solid lubricant, an excessive wear may be generated. In particular, this phenomenon is conspicuous in starter brushes to which Pb has been added. When Pb and the metal sulfide solid lubricant are eliminated simultaneously, the service life of the brush will be reduced significantly. Hence in many cases, the metal sulfide solid lubricant can not be eliminated from Pb-less brushes.
The present inventors estimated the mechanism by which the metal sulfide solid lubricant the accelerates oxidization of the copper powder and the embedded lead under high humidity as follows: At the time of sintering the brushes, sulfur is liberated from the metal sulfide solid lubricant added to the brush and sulfur adsorbs on the surface of copper to produce copper sulfide. If moisture acts on copper sulfide under high humidity, strongly acidic copper sulfate will be produced to corrode severely the copper powder and the lead.
The mechanism by which Pb prevents the oxidization of the copper powder and the embedded lead in the brush is not known exactly. The present inventors estimate that Pb contained in the brush partially evaporates at the time of sintering and coats the surface of copper in the form of a very thin Pb layer. And this Pb layer protects the inner copper from sulfate ion, etc.
The present inventors searched for materials which can prevent, in place of Pb, the increases in the lead connection resistance and the resistivity of the brush body under high humidity. Only indium was found to be effective in preventing the increases in the lead connection resistance and the resistivity of the brush body under high humidity. According to the present invention, indium is added at least to the interface between the brush body and the lead, and the increase in the lead connection resistance in high humidity can be prevented.
According to the present invention, indium is added substantially in all over the brush body, and the increase in the resistivity of the brush body as well as the increase in the lead connection resistance can be prevented. When the indium concentration is from 0.4 to 8 wt %, the increase in the lead connection resistance and the increase in the resistivity can be reduced sufficiently.
According to the present invention, as indium is locally added in the neughborhood of the lead to be embedded, the use of indium can be held down.
Moreover, according to the present invention, as indium is fed from the lead, the use of indium can be held down.
As for the metal sulfide solid lubricant, for example, molybdenum disulfide or tungsten disulfide is used. When its addition is from 1 to 5 wt %, good lubrication can be obtained.
Prevention of oxidation caused by the metal sulfide solid lubricant is particularly significant when the non-electroplated copper lead, which is prone to oxidization, is used for the lead.